1Center for Gene Regulation in Health and Disease, Department of Biological, Geological, and Environmental Sciences, Cleveland State University Cleveland, OH, USA.

Abstract

In a number of microbial pathogens that undergoes antigenic variation to evade the host's immune attack, genes encoding surface antigens are located at subtelomeric loci, and recent studies have revealed that telomere components play important roles in regulation of surface antigen expression in several of these pathogens, indicating that telomeres play critical roles in microbial pathogen virulence regulation. Importantly, although telomere protein components and their functions are largely conserved from protozoa to mammals, telomere protein homologs in microbial pathogens and humans have low sequence homology. Therefore, pathogen telomere components are potential drug targets for therapeutic approaches because first, most telomere proteins are essential for pathogens' survival, and second, disruption of pathogens' antigenic variation mechanism would facilitate host's immune system to clear the infection.

(A)EPA1–7 are located at subtelomeric loci in C. glabrata. The positions of seven EPA genes at their respective chromosome end loci are shown. EPA1 is furthest away from the telomere and is the only one that is expressed normally, while EPA 2–7 are usually silenced by TPE. Pink arrowheads, telomere repeats. (B) The telomere protein complex in budding yeast. Rap1 is the duplex telomere DNA binding factor, while Cdc13/Stn1/Ten1 binds to single-stranded telomere DNA. Rap1 recruits Sir3 and Sir4, which in turn recruits Sir2. Sir3 and Sir4 can also interact with histones directly. Sir2’s deacetylase activity maintains the hypoacetylated state of histones. Rap1 also recruits Rif1 and Rif2. Red stars, histone acetylation groups; green cylinders, nucleosomes.

(A) The organization of subtelomere elements in P. falciparum. Immediately internal to the telomere tract are six telomere-associated repeat elements (TAREs 1–6), with the largest one, rep20, located furthest away from the telomere repeats. One or two var genes are usually found immediately upstream of rep20, followed by the rifin, stevor, and Pf60 gene families. Depending on the upstream flanking sequences, three classes of var genes have been identified. The ones with associated UpsB and UpsA are located at subtelomeric regions and transcribed in opposite directions as drawn, while the ones with associated UpsC are located as gene arrays inchromosome-internal loci (B). Little is known about the telomere proteins in Plasmodium, except that Sir2 and Orc1 is located at the telomere vicinity (Mancio-Silva et al., 2008), and HP1 is associated with subtelomeric TAREs (Perez-Toledo et al., 2009), which are shown in the bottom diagram in(A).

Distribution of VSG genes in T. brucei genome. (A) In a bloodstream form VSG expression site (B-ES), the VSG gene is the last one in the large polycistronic transcription unit and is located within 2kb of the telomere repeats. A stretch of 70bp repeats with various length is located upstream of the VSG gene followed by a number of ES associated genes (ESAGs). (B) The metacyclic VSG expression site (M-ES) is a monocistronic transcription unit also located at subtelomeric region. (C) Most VSG genes and pseudogenes (and some ESAG genes) are found in gene arrays located at subtelomeric regions on megabase chromosomes. Short stretches of 70bp repeats are found upstream of each gene. (D) On minichromosomes, single VSG genes and upstream 70bp repeats are also found at subtelomeric regions. (E) The telomere protein, TbRAP1, has been shown to play an important role in silencing subtelomeric VSG genes. TbTRF and TbRAP1 are two known T. brucei telomere proteins. TbRAP1-mediated silencing is stronger (thick line) at telomere-proximal VSG locus and weaker (thin line) at telomere-distal ES promoter region. Several factors important for ES promoter silencing are also shown.

VSG switching can occur through in situ switch, gene conversion, or crossover. Top Middle, before switching, an active B-ES (long red arrow), a silent B-ES (short blue arrow), a VSG gene at a minichromosome subtelomere, and an array of VSG genes and pseudogenes on a megabase chromosome are shown. In situ switch (top left) results from turning on (long blue arrow) of the silent B-ES and turning off (short red arrow) of theactive B-ES simultaneously without any DNA rearrangements. In gene conversion, a silent VSG gene is duplicated into the active B-ES, and the originally active VSG gene is lost. The VSG donor can come from a silent B-ES (bottom left), a minichromosome subtelomere (bottom middle), or a VSG gene array (bottom-right). In VSG cross-over (top right), the active VSG and a silent VSG (most often from a silent B-ES) exchange their loci reciprocally, resulting in a new VSG gene in the active B-ES without losing any genetic information. The cross-over site is often found within the 70bp repeats upstream of the VSG genes, although it can locate more upstream, and all B-ESs have high sequence homology.

Gene arrays at the ends of three Pneumocystis carinii chromosomes. MSG genes (cyan colored arrows) are located closest to the telomere and subtelomeric repetitive sequences. A single copy UCS is found in the active MSG expression site immediately upstream of the MSG gene.

The organization of vlsE and the array of silent vls genes on lp28 linear plasmid of B. burgdorferi. The vls1 gene (moss colored arrow) is expressed from the vlsE expression site next to the telomere (pink arrows). The direct repeats (barred boxes) and the unique regions (green boxes) flanking the vls1 gene and the lipoprotein leader sequence (rouge box) upstream of vls1 are marked. The silent array includes vls2–16 genes (various colored arrows) going to the opposite direction from vls1 are located at the internal region of the linear plasmid.